Real-Time Rate Limiter

You are tasked with implementing a rate limiter that controls the number of requests a user can make within a given time window. The rate limiter should support the following requirements:

• It should accept requests coming in with a user identifier (e.g., user ID or API key) and a timestamp.
• Each user is allowed to perform up to N requests in a window of T seconds. Both N and T should be configurable.
• If a user exceeds the allowed number of requests within the window, additional requests should be denied until the window resets.
• The solution should be efficient in both time and space, considering that in a real-world scenario, there could be many users making requests at the same time.

Define the interface for your rate limiter, implement the functionality, and demonstrate its correctness with sample inputs that simulate users making requests. Think about edge cases (such as burst requests at the window boundary) and ensure your solution handles them appropriately.

Feel free to use any programming language of your choice.